1. Field of the Invention
This invention relates to a light sensitive element signal transfer circuit having amplification means. The circuit may be used in a scanner environment wherein alternate elements of a linear array of the light sensitive elements are coupled to one of two video output lines. The array is formed as integrated circuits in a semiconductor substrate.
2. Description of the Prior Art
Light sensitive device arrays formed in integrated semiconductor circuits, e.g., as an array of photo-diodes, use diffusions providing p-type or n-type regions in an n-type or p-type silicon substrate, respectively, to collect holes or electrons generated by electromagnetic radiation on or around the diffused region. The charge so collected depends on the intensity and time of exposure of the diffused region to the radiation. The size of the diffused regions is often a limiting factor in providing suitable arrays since the charge varies directly as the area of the diffused regions. When a reduced exposure time or faster integration time is required, the area of the diffused region must be reduced to maintain a given voltage swing, e.g., on the photo-diode. The variation in charge is equal to the capacitance of the photo-diode times the voltage swing and is also equal to the intensity of the radiation times the time of exposure times the diffused area times optical constants. As is known, the photo-diode capacitance is due to the diffusion capacitance and the effective diffused area is the area of the optical window at the diffused region.
In order to transfer signals from the photo-diodes to signal processing circuits, a substantial amount of change in charge had to be produced and, therefore, large photo-diodes and long exposure times were required.
In Electronics, May 1, 1967, pp. 75-78, there are described light sensitive arrays utilizing devices operating in the storage mode which sum the light or radiation falling on the diffused region or junction over a period of time, with readout being provided in a relatively short time interval. In U.S. Pat. No. 3,770,967, filed Feb. 24, 1972, and having a common assignee, there is disclosed a light sensitive element coupled to an amplifying circuit for transfer of signals to appropriate signal processing circuits. This circuit arrangement is suitable for light detection but it employs direct current coupling and does not use light integration or charge accumulation techniques. In U.S. Pat. No. 3,814,846, filed Jan. 20, 1972, there is disclosed a self-scanning photo-detection array wherein the light sensitive devices are arranged in a row, with alternate devices being sequentially sensed and coupled directly to one of two video output lines. The signals produced at the two video lines of this array have a relatively low magnitude, and, consequently, a low signal to noise ratio.
Transistor circuits are known, e.g., in U.S. Pat. Nos. 3,506,851, filed Dec. 14, 1968, and 3,631,267, filed June 18, 1970, which employ bootstrapping techniques for compensating the threshold voltage drop across output devices for logic circuits, but circuits are not known for analog circuits which are capable of eliminating or minimizing the threshold voltage loss in a light sensitive array which includes capacitors associated with light sensitive devices and analog signals such as photo-diodes.